"This was such a bizarre concept to me—that all this supposedly `healthy' exercise was actually detrimental to my heart," says Fanselow. "To this day, I still have trouble getting used to it."

Why is AFL a bad thing?

If you experience AFL during a race or training run, the arrhythmia could degrade into a more serious condition, such as ventricular fibrillation, a lethal situation. Furthermore, running with AFL may compromise an athlete's ability to pump blood to working muscle and induce undue fatigue.

At all costs, if your heart rhythm suddenly feels funny and you become unusually fatigued, stop exercise or exertion immediately and see or contact your physician, unless you feel an ER visit is warranted.

Is atrial flutter treatable?

Yes. Treatment is normally radiofrequency ablation (i.e. destruction) of the irritable area of the atrium giving off the pesky, rapid beats. The ablation procedure involves shoving catheters through both femoral (groin) veins and into the right atrium, then using specialized probes inserted through the catheters to ablate the petulant tissue. The procedure, which can take up to five hours, has over a 90-percent success rate. Eighty to 90 percent of patients opt for the ablation over drug therapy, the latter of which may become chronic and have side effects, according to Dr. Oza.

"Reoccurrence of atrial flutter is not common post ablation," says Dr. Oza. "However, a patient is always at risk for atrial fibrillation, a higher-rate atrial arrhythmia, because of risk factors that are still present after the atrial flutter is treated."

In other words, in the case of Fanselow, who was still contemplating ablation after a second AFL episode in July, right after winning Colorado's North Fork 50-miler, he may be at risk for atrial fibrillation post treatment for AFL. Of note, Fanselow completed the Collegiate Peaks 50-mile race in Buena Vista, Colorado, in May (his first ultra marathon) with no arrhythmia problems, finishing second overall, and continuing to train hard leading up to North Fork, where he realized the AFL was back and at a greater rate upon crossing the finish line. He ambled directly into the North Fork medical tent while remembering that he also felt oddly crappy the last five miles. Fortunately, the AFL again converted back to a normal rhythm without intervention, this time about one hour post run.

"A week later I was back in Dr. Oza's office discussing the latest episode and my options. He was now very concerned about me continuing to run hard, given my propensity for flutter, and particularly my inability to detect its onset during intense exercise," says Fanselow. Additionally, Fanselow had been wrestling with the notion and medical advice to stop hard endurance training altogether to decrease the likelihood of reoccurring AFL episodes.

Fanselow consulted with his wife and scheduled the AFL ablation procedure 17 days prior to his third 50-mile race, Run Rabbit Run in Steamboat Springs, Colorado, on September 18.